From dcf233230158d6d2bc96d73dfd3566d998cf92d1 Mon Sep 17 00:00:00 2001
From: Brad Campbell <bradjc5@gmail.com>
Date: Wed, 3 Jul 2024 15:26:37 -0400
Subject: [PATCH] store two most recent syscalls

---
 kernel/src/process_loading.rs  |  2 +-
 kernel/src/process_standard.rs | 29 ++++++++++++++++++++++-------
 2 files changed, 23 insertions(+), 8 deletions(-)

diff --git a/kernel/src/process_loading.rs b/kernel/src/process_loading.rs
index 54044eb493..705836cf68 100644
--- a/kernel/src/process_loading.rs
+++ b/kernel/src/process_loading.rs
@@ -375,7 +375,7 @@ fn load_process<C: Chip>(
     // get a process and we didn't get a loading error (aka we got to
     // this point), then the app is a disabled process or just padding.
     let (process_option, unused_memory) = unsafe {
-        ProcessStandard::<C, crate::process_standard::ProcessStandardDebugFull>::create(
+        ProcessStandard::<C, ()>::create(
             kernel,
             chip,
             process_binary,
diff --git a/kernel/src/process_standard.rs b/kernel/src/process_standard.rs
index 3e62a4ad40..20b231f782 100644
--- a/kernel/src/process_standard.rs
+++ b/kernel/src/process_standard.rs
@@ -52,7 +52,7 @@ pub trait ProcessStandardDebug {
     fn set_new_app_stack_min_pointer(&self, ptr: *const u8);
 
     fn set_last_syscall(&self, syscall: Syscall);
-    fn get_last_syscall(&self) -> Option<Syscall>;
+    fn get_last_syscall(&self, index: usize) -> Option<Syscall>;
     fn reset_last_syscall(&self);
 
     fn increment_syscall_count(&self);
@@ -104,6 +104,10 @@ struct ProcessStandardDebugFullInner {
     /// What was the most recent syscall.
     last_syscall: Option<Syscall>,
 
+    /// What was the previous most recent syscall. We store two syscalls because
+    /// often when debugging the most recent syscall is `Yield`.
+    penultimate_syscall: Option<Syscall>,
+
     /// How many upcalls were dropped because the queue was insufficiently
     /// long.
     dropped_upcall_count: usize,
@@ -159,13 +163,23 @@ impl ProcessStandardDebug for ProcessStandardDebugFull {
     }
 
     fn set_last_syscall(&self, syscall: Syscall) {
-        self.debug.map(|d| d.last_syscall = Some(syscall));
+        self.debug.map(|d| {
+            d.penultimate_syscall = d.last_syscall;
+            d.last_syscall = Some(syscall);
+        });
     }
-    fn get_last_syscall(&self) -> Option<Syscall> {
-        self.debug.map_or(None, |d| d.last_syscall)
+    fn get_last_syscall(&self, index: usize) -> Option<Syscall> {
+        self.debug.map_or(None, |d| match index {
+            0 => d.last_syscall,
+            1 => d.penultimate_syscall,
+            _ => None,
+        })
     }
     fn reset_last_syscall(&self) {
-        self.debug.map(|d| d.last_syscall = None);
+        self.debug.map(|d| {
+            d.penultimate_syscall = None;
+            d.last_syscall = None;
+        });
     }
 
     fn increment_syscall_count(&self) {
@@ -210,6 +224,7 @@ impl Default for ProcessStandardDebugFull {
                 app_stack_min_pointer: None,
                 syscall_count: 0,
                 last_syscall: None,
+                penultimate_syscall: None,
                 dropped_upcall_count: 0,
                 timeslice_expiration_count: 0,
             }),
@@ -241,7 +256,7 @@ impl ProcessStandardDebug for () {
     fn set_new_app_stack_min_pointer(&self, _ptr: *const u8) {}
 
     fn set_last_syscall(&self, _syscall: Syscall) {}
-    fn get_last_syscall(&self) -> Option<Syscall> {
+    fn get_last_syscall(&self, _index: usize) -> Option<Syscall> {
         None
     }
     fn reset_last_syscall(&self) {}
@@ -1318,7 +1333,7 @@ impl<C: Chip, D: 'static + ProcessStandardDebug + Default> Process for ProcessSt
     }
 
     fn debug_syscall_last(&self) -> Option<Syscall> {
-        self.debug.get_last_syscall()
+        self.debug.get_last_syscall(0)
     }
 
     fn get_addresses(&self) -> ProcessAddresses {